This invention relates to superconductor systems, and in particular to crystalline structures having superconducting properties.
Theoretical and experimental research in the field of superconducting materials by thousands of researchers has led to the discovery of a variety of oxide compounds which become superconducting at relatively high temperatures (T.sub.c), i.e., above about 20.degree. K. The widely known high temperature superconductors are oxides, and presently contain (1) copper and/or bismuth, (2) barium or other alkaline earths such as strontium or calcium, and (3) trivalent elements such as yttrium. Rare earth elements having atomic numbers ranging from 57 to 71 (lanthanum to lutecium), are substituted for yttrium in some materials, as are thallium or bismuth. Representative of superconductors are the following:
(1) oxide materials containing lanthanum, strontium and copper, bearing the formula La.sub.2-x Sr.sub.x CuO.sub.4, commonly referred to as L-S-C-O, and recently discovered variants thereof such as materials in which La substituted with, for example, praseodymium, neodymium, uranium, thorium, cerium and others containing a tetravalent ion in place of Sr; PA1 (2) oxide materials containing yttrium, barium and copper, bearing the formula YBa.sub.2 Cu.sub.3 O.sub.7-.delta., commonly referred to as 1-2-3 (rare earth elements can be substituted for yttrium, and the resulting compounds are also superconducting). Other Y-Ba-Cu materials include 1-2-4 and 2-4-7; PA1 (3) oxide materials containing bismuth, strontium, calcium and copper, bearing such formulas as Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8+x and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10+x, commonly referred to as B-C-S-C-O, and related materials including those in which Pb and copper replace Bi; PA1 (4) oxide materials containing thallium, barium, calcium and copper, bearing such formulas as Tl.sub.2 Ba.sub.2 CuO.sub.x, Tl.sub.2 CaBa.sub.2 Cu.sub.2 O.sub.x, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x, Tl.sub.2 Ca.sub.3 Ba.sub.2 Cu.sub.4 O.sub.x, TlCaBa.sub.2 Cu.sub.2 O.sub.x, and TlCa.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x, commonly referred to as T-C-B-C-O, and related materials including those in which Pb and/or Cu replace Tl; and PA1 (5) oxide materials containing bismuth, barium, potassium and copper, bearing the formula Ba.sub.1-x K.sub.x BiO.sub.3, identified as B-K-B-O.
Copending application Ser. No. 263,750 discloses certain improved superconductor materials and methods of manufacture thereof, and is incorporated herein by reference for all purposes. See also Morris et al., "Eight New High Temperature Superconductors With the 1:2:4 Structure", Phys. Rev., 39, 7347 (April, 1989), which is also incorporated herein by reference for all purposes.
Introduction of defects in intermetallic type II superconductors was proposed to increase their critical current density. See, for example, Campbell et al., "Pinning of Flux Vortices in Type II Superconductors," Phil. Mag., 18, 313 (1968). Thermally activated flux creep has also been recognized as a problem with high-temperature superconductors.
However, in the case of high-temperature superconductors, the introduction of defects to increase critical current density to a useful level has met with only limited success. For example, in Gammel et al., Phys. Rev. Lett., 59, 2592 (1987), an increased density of twin boundaries provides only moderate improvement in flux pinning. Some increase in low temperature J.sub.c in YBa.sub.2 Cu.sub.3 O.sub.7 in strong magnetic fields was achieved by the introduction of point defects by neutron irradiation in, for example, Willis et al., "Radiation Damage in YBa.sub.2 Cu.sub.3 O.sub.7-x By Fast Neutrons", High Temperature Superintroductors, MRS Symposium Proceedings Vol. 99, 391-94 (1988). However, even in Willis et al., the increase in J.sub.c was limited and at 7.degree. K. and B=4T increased to only about 10.sup.4 A/cm.sup.2 after about 10.sup.18 n cm.sup.-2 above which value superconductivity was adversely effected by the neutron dose. This may limit the wide application of neutron irradiation to provide improvement in flux pinning. Critical currents in polycrystalline high-temperature superconductors are still further reduced by weak links at the grain boundaries, which are made worse by high porosity, misalignment of the crystalline axis of adjacent grains, and by formation and accumulation of non-superconductor phases (compounds) at boundaries between superconducting grains.
The need for additional high temperature superconductors and methods of manufacturing superconductors is great, not only to achieve superconductors with higher T.sub.c 's, but also to achieve superconductors with improved J.sub.c 's in magnetic fields, improved mechanical properties, stability, and ease of processing.